1. Field of the Invention
The present invention relates to a moving image information coding apparatus, and more particularly to an image processing method and image information coding apparatus using the same relating to memory access in a motion search device.
2. Description of the Related Art
Image information coding apparatus are available which are compliant with MPEG and other similar schemes. In these schemes, image information is treated as digital information. Image information is compressed through orthogonal transform such as discrete cosine transform and motion compensation by taking advantage of redundancy which is a typical characteristic of image information. This is done to ensure high efficiency in transmission and storage of information.
Moving image coding schemes such as MPEG are predictive coding schemes using motion compensation. As a result, motion vectors must be detected for motion compensation in the coding process. In order to achieve high efficiency in coding, motion vectors must be detected in an as wide search range as possible. However, motion search in a wide range requires an enormous number of computations and enormous amount of data.
In order to resolve such a problem, a hierarchical motion vector search is conducted. FIG. 7 illustrates an original screen, reference screen and hierarchical vector.
A description will be given below of a hierarchical motion vector search. An input image and reference image are reduced at the same reduction ratio to the same resolution. A coarse motion search is performed using the reduced images. The coarse motion vector obtained here will be hereinafter referred to as a hierarchical vector.
Then, a motion search is performed in a small area around the hierarchical vector in the original image resolution plane (this motion search will be hereinafter written as refinement).
It is common to conduct a search in a fixed search area (±X,±Y) appropriate to the capability of the refinement device by transferring reference image data from the frame memory.
Japanese Patent Laid-Open No. 2007-274383 discloses that, during the transfer of the reference image data indicated by the motion vector from the image memory, it is determined whether any part of the reference image lies off the image frame, and that this determination is made in word units.
However, Japanese Patent Laid-Open No. 2007-274383 does not disclose that the search area of the memory is accessed or searched in word units in the refinement using a hierarchical vector.